This invention relates in general to electrically conductive layers and, more specifically, to novel electrically conductive devices and process for using the devices.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind an electrostatic charge pattern in the nonilluminated areas. This resulting electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer sandwiched between the contiguous charge transport layer and a supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the anode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers (CGL) and charge transport layers (CTL) have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain diamine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed, for example, in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles to form a toner image. During cycling of these photosensitive members, it is desirable to expose the photoreceptor to activating radiation prior to transfer and prior to cleaning. Exposure from the toner image (or residual image prior to cleaning) side of the photoreceptor is less desirable than from the back side of the photoreceptor because the toner image interferes with complete exposure of the underlying parts of the photoreceptor, i.e. a shadow effect, so that discharge of the photoreceptor is less complete in the areas underlying the toner than in areas not covered by toner.
Erasure exposure of selected unexposed portions of the photoreceptor prior to development is often desirable to prevent dense deposits of toner from forming along the edges of the photoreceptor, between documents, and along document margins, because such deposits are difficult to clean, cause toner waste, and, in some cases form dark toner bands on the final printed document. Although these types of erase exposure can be carried out with light sources positioned along the outer surface of a photoreceptor, the light sources greatly limit machine design because the presence of the light sources interferes with placement of other processing stations such as charge, development, transfer, paper stripping, and cleaning stations. Thus, placement of sources of activating radiation on the rear or backside of the photoreceptor is highly desirable. However, when ground planes containing conductive particles dispersed in a resin binder are used in photoreceptors, difficulties can be encountered with non-uniform dispersion of the conductive particles in the binder. Agglomerates and other non-uniform dispersions of the conductive particles adversely affect the quality of the electrostatic charging, development, transfer and discharging cleaning processes. Moreover, this type of ground plane tends to be opaque to light so that erasure from the rear surface is impossible, impractical or of poor quality.
Also, with ground planes containing conductive particles dispersed in a resin binder, difficulties can be encountered with migration of the resin binder and/or conductive particles into subsequently applied layers that contain solvents which at least partially dissolve the resin binder in the conductive layer. Such migration of the resin binder or conductive particles can adversely affect the integrity of the ground plane and the electrical properties of the ground plane and/or the subsequently applied layers. More specifically, polymers in the binders utilized for ground planes can migrate into the charge generating layer and cause charge trapping. When charge trapping occurs during cycling, internal fields build up and background prints out in the final printed copies. Further, conductive particles can move up to subsequently applied layers and prevent the photoreceptor from receiving a full electrostatic charge in the areas where the conductive material migrated. For example, migration of conductive particles such as carbon black into subsequently applied layers causes lower charge acceptance and perhaps V.sub.R cycle-up. The regions of lower charge acceptance appear as white spots in the final printed copy. Solvent attack can also cause discontinuities in the ground plane resulting in non-uniform charging which ultimately causes the formation of distorted images in the final toner image. Cross-linking of the resin binder in the ground plane reduces solubility. However, existing methods of cross-linking polymers such as hydroxylic polymers, although chemically efficient in the cross-linking process itself, leave much to be desired in applications for photoreceptors because of catalytic or process residues which can permanently reside in the photoreceptor. Such residues, even at the parts per million level, are very often deleterious to one or more of the sensitive electrical properties required for superior photoreceptor performance.